Process for Removal of Sulfur Oxides From Waste Gases

This invention relates to processes for removal of sulfur oxides (SOx) in waste gases with the use of magnesium hydroxide as a principal absorbent. More specifically, the invention relates to a process of this character wherein desulfurization is carried cut with magnesium hydroxide as an absorbent, and the SOx thus absorbed is recovered as gypsum of good quality.

The process of causing an aqueous slurry for absorbing of a calcium compound such as calcium hydroxide or calcium carbonate to contact a waste gas containing SOx in an absorption tower thereby to fix the SOx as calcium sulfite and thereby to carry out desulfurization is known. If this calcium sulfite is oxidized, gypsum will be obtained.

Since this process is accompanied by a number of problems, we have previously proposed an improved process as disclosed in the specification of Japanese Patent Application No. 30279/1973, Japanese Patent Publication No. 11680/1977, wherein magnesium hydroxide is jointly used as an absorbent in a specific proportion. This magnesium hydroxide also reacts with the SOx to become magnesium sulfite, which is oxidized at the same time as the oxidation of the calcium sulfite to become magnesium sulfate. This calcium sulfate is caused to react with the magnesium hydroxide and becomes magnesium hydroxide by a double decomposition as indicated by the equation set forth below, and, at the same time, gypsum is formed. ##STR1##

In a preceding invention of ours, the mole ratio of Ca(OH).sub.2 /Mg(OH).sub.2 in the absorbing slurry is limited to 0.5 to 99. The reason for this is as follows.

Since a magnesium compound has a higher SOx absorbing efficiency than a calcium compound, it would appear that a small value of this mole ratio is desirable if only the desulfurization efficiency were to be considered. However, a small value of this mole ratio gives rise to a large quantity of the Ca(OH).sub.2 consumed in the reaction indicated by the above reaction equation of the supplemented Ca(OH).sub.2 and, consequently, results in the formation of a large quantity of CaSO.sub.4 in the Mg(OH).sub.2 regeneration reactor. However, by a reaction according to the reaction equation (1), only a coarse or crude gypsum in muddy state can be obtained. As a consequence, the quantity of the crude gypsum in muddy state formed in the Mg(OH).sub.2 regeneration reactor becomes considerably more than the quantity of gypsum formed in the subsequent oxidation step, and this results in a lowering of the quality of the recovered gypsum.

In order to solve this problem arising from the contradictory nature of improvement of absorptivity of the absorption liquor and impairment of quality of the gypsum produced, we have previously invented a waste gas desulfurization method wherein, by limiting the mole ratio of Ca(OH).sub.2 /Mg(OH).sub.2 in the absorption liquor supplied to the absorption tower to 0.5 to 99, the absorption efficiency of magnesium compounds is sustained, whereby gypsum of good quality is obtained, as disclosed in the Japanese Patent Publication hereinbefore.

As mentioned hereinbefore, however, a magnesium compound such as Mg(OH).sub.2 has a higher SOx absorption efficiency than a calcium compound such as Ca(OH).sub.2. Accordingly, it would be highly profitable if, by using a greater quantity of Mg(OH).sub.2 than Ca(OH).sub.2, preferably by using substantially only Mg(OH).sub.2, for the absorbent, a gypsum of good quality could be obtained.

It is an object of this invention to provide a solution for the above stated problem.

Another object of this invention is to realize improvement of desulfurization efficiency and production of gypsum of good quality by using magnesium hydroxide containing substantially no calcium hydroxide for the SOx absorbent.

The process for removal of sulfur oxides from waste gases containing the sulfur oxides in accordance with the present invention, in the broadest aspect thereof, comprises the steps in combination of:

A. contacting an aqueous slurry containing magnesium hydroxide as well as gypsum with a waste gas containing sulfur oxides thereby to fix the sulfur oxides as magnesium sulfite;

B. contacting the aqueous slurry containing the magnesium sulfite as well as the gypsum with an oxidizing agent thereby to convert the magnesium sulfite into magnesium sulfate;

C. subjecting the aqueous slurry containing magnesium sulfate as well as gypsum to filtration, thereby to obtain an aqueous solution of the magnesium sulfate and a cake of the gypsum;

D. causing the magnesium sulfate in the aqueous solution to undergo double decomposition with calcium hydroxide in the presence of a quantity of gypsum crystals as seed crystals thereby to obtain a slurry of magnesium hydroxide and gypsum, which slurry is to be recycled to the step A.

In accordance with this invention, a magnesium compound such as magnesium hydroxide is used in a great quantity relative to that of a calcium compound such as calcium hydroxide thereby to improve the absorption efficiency of the absorbing slurry. For this purpose, the mole ratio of Mg compound to the Ca compound to be supplied to the Mg(OH).sub.2 regeneration reactor, that is, the mole ratio of the double decomposition components, is caused to be MgSO.sub.4 /Ca(OH).sub.2 .gtoreq.1, and we have carried out studies for extending the invention to a process wherein, for the constituent of the absorbent of the absorption liquor, it is possible even to use substantially only Mg(OH).sub.2 without any Ca(OH).sub.2. It this we have finally succeeded.

In accordance with this invention, by adding crystalline gypsum as seed crystals in the Mg(OH).sub.2 regeneration reaction step wherein MgSO.sub.4 is caused to undergo double decomposition by adding Ca(OH).sub.2, the absorptivity of the Mg compound is caused to be amply exhibited, and, at the same time, gypsum of good quality is obtained, without limiting the mole ratio Ca(OH).sub.2 /Mg(OH).sub.2 in the absorption liquor supplied to the absorption tower. This process of the invention is particularly effective in the case where the mole ratio MgSO.sub.4 /Ca(OH).sub.2 in the double decomposition in the Mg regeneration reaction step is equal to or greater than 1 (unity), that is, where the Ca(OH).sub.2 in the absorption liquor supplied to the absorption tower is of zero quantity.

The nature, utility, and further features of this invention will be apparent from the following detailed description beginning with a consideration of general aspects of the invention and concluding with a specific example of practice including a number of test runs.

In the illustrations

FIG. 1 is a flow sheet showing one example of practice of the process according to this invention; and

FIGS. 2 through 6 are photomicrographs (magnification 150.times.) respectively of gypsum produced in the test runs in the example.

The invention will first be described with respect to one example of practice thereof as indicated in the flow sheet of FIG. 1. This process comprises an Mg(OH).sub.2 regeneration step I, a waste gas absorption step II, an MgSO.sub.3 oxidation step III, and a gypsum separation step IV. In the process step I, the double decomposition of the MgSO.sub.4 formed in the step III and Ca(OH).sub.2 is carried out, and the gas absorption of the step II is carried out with the slurry formed in this step I containing the Mg(OH).sub.2 and the gypsum. On one hand, the gypsum formed in the step I and passing through the steps II and III is separated in the step IV.

In this Mg(OH).sub.2 regeneration reaction step I, the filtrate containing MgSO.sub.4 from which gypsum has been removed by separation in the gypsum separation step IV is supplied into a regeneration reactor 1, into which Ca(OH).sub.2 is added, and Mg(OH).sub.2 is regenerated by the double decomposition reaction indicated in the reaction equation given hereinbefore. In this case, an Mg(OH).sub.2 slurry containing gypsum of good quality is formed by the addition of gypsum seed crystals in accordance with this invention. This slurry and the absorption liquor are supplied to an absorption tower 2.

As mentioned hereinbefore, a greater proportion of Mg(OH).sub.2 in the mole ratio Ca(OH).sub.2 /Mg(OH).sub.2 in the absorption slurry supplied to the absorption tower 2 results in a higher absorptivity, and in particular an absorption liquor produced from the regeneration of the Mg(OH).sub.2 by carrying out the double decomposition in the case of only Mg(OH).sub.2 without the presence of Ca(OH).sub.2 as the absorbent in the absorption liquor, that is, in the range of a double decomposition constituent mole ratio MgSO.sub.4 /Ca(OH).sub.2 equal to or greater than 1 (unity), is very suitable; and the quantity of MgSO.sub.4 supplied into the Mg(OH).sub.2 regeneration reactor 1 is regulated so as to maintain the double decomposition mole ratio within this range.

It is preferable that the concentration of the aqueous magnesium supplied to the double decomposition reaction, that is, that in the slurry containing gypsum after the oxidation step or in the aqueous solution after the separation of gypsum be of the order of 2 to 10 percent. On one hand, the quantity of the Ca(OH).sub.2 supplied is ordinarily substantially the same as that of the SOx to be absorbed.

The calcium hydroxide, which is one of the reactants in the double decomposition, may be introduced as calcium hydroxide into the double decomposition reactor, or it may be introduced thereinto in the form of calcium oxide and caused in situ to form calcium hydroxide. In the case where it is introduced in the form of calcium hydroxide, it can be introduced in the form of a slaked product of calcium oxide. The slaking water in this case may be pure water or it may be the slurry obtained from the double decomposition reactor.

The Ca(OH).sub.2 to be fed to the Mg(OH).sub.2 regeneration reactor may be in a dry particulate form or in a slurry form, and when it is in a slurry form the concentration of the slurry is preferably of the order of 10 to 60%, preferably of the order of 20 to 40%, from a viewpoint of industrial or commercial operation.

One other requisite condition according to this invention in carrying out the double decomposition reaction is that it be carried out in the presence of gypsum seed crystals. In the Mg(OH).sub.2 regeneration reactor 1, Mg(OH).sub.2 and CaSO.sub.4.2H.sub.2 O are regenerated by the addition of Ca(OH).sub.2 according to Eq.(1) set forth hereinbefore. In this case, the addition of the seed crystals causes the CaSO.sub.4.2H.sub.2 O to be formed, not in a muddy state, but as crystals of excellent quality and of large crystal form.

Principal examples of possible sources of seed crystals of gypsum, in addition to a portion of the gypsum separated in a separator 5 are the gypsum in slurry form withdrawn out of the bottom of a thickener 4, the slurry taken out of an oxidizing tower or reactor 3, and slurry taken out of the absorption tower or reactor 2. That is, any gypsum formed in the process may be used for this purpose, or crystalline gypsum separately prepared may be added.

There is an optimum value of the quantity of the crystalline gypsum added as seed crystals, desirably being 30 to 150 percent by weight, preferably 80 to 120 percent by weight, of the quantity of gypsum formed in the double decomposition step.

While there is no particular limit to the temperature of the Mg(OH).sub.2 regeneration reaction step, we have verified, as a result of various tests, that a high temperature results in the formation of gypsum of good quality of plate form and large size. Accordingly, the temperature range which can be used in the practice of this invention is, in general, from room temperature to 85.degree. C. and, preferably, from 40.degree. to 80.degree. C. The pressure under which this step is carried out may be atmospheric pressure or a higher pressure. There is no particular necessity of limiting the pH.

An absorption liquor comprising a slurry containing gypsum and Mg(OH).sub.2 from the above described Mg(OH).sub.2 regeneration step is accommodated in the absorption tower or reactor 2. Into this absorption tower 2, the waste gas 7 is introduced from a suitable point such as, for example, the top of the tower thereby to cause absorption of SOx and formation of MgSO.sub.3. This absorption operation can be carried out by any process which can be applied to scrubbing of a gaseous component by a liquid or, more specifically, by an aqueous slurry. Furthermore, the absorption liquor contains Mg(OH).sub.2 as well as gypsum and may contain a small quantity of another SOx absorbent provided that this Mg(OH).sub.2 functions as a substantial absorbent. Examples of such other absorbents are magnesium carbonate, dolomite, and other magnesium compounds which are added in some cases to make up the consumed Mg(OH).sub.2 (the case wherein the consumption of Mg(OH).sub.2 is compensated for by adding Mg(OH).sub.2 being, of course, within the purview of this invention), or calcium hydroxide, calcium carbonate, and other calcium compounds which may, if desired, be used. The absorption pressure and temperature are ordinarily of the order of -300 to +300 mm H.sub.2 O, gage and of the order of 40.degree. to 60.degree. C., respectively.

The slurry containing magnesium sulfite formed in this manner is oxidized in the absorption tower 2 by the oxygen in the waste gas or is sent to the oxidation tower or reactor 3 and is there oxidized into MgSO.sub.4 by a suitable oxidizing agent, preferably molecular oxygen, e.g., air or oxygen gas introduced as at 8. This oxidation can be carried out by any process in which ample contact between the slurry and the oxidizing agent used in assured, but, ordinarily, it is carried out by blowing into the slurry a gaseous oxidizing agent, e.g., molecular oxygen, under the conditions of a pressure of the order of 0.3 to 3.0 kg./cm.sup.2., gage, and a temperature of the order of 50.degree. to 80.degree. C.

Because of its high solubility, the MgSO.sub.4 is in the liquid phase, whereas, the gypsum is mostly in the solid phase because of its low solubility. Accordingly, the slurry after the above described oxidation step is subjected to a suitable solid-liquid separation or is supplied to a filtering device such as, for example, a thickener 4, thereby to concentrate the gypsum by sedimentation. The gypsum is further collected by the separator 5, and the solution portion containing MgSO.sub.4 is recirculated through pipe lines 10 and/or 11 to the regeneration reactor 1. One portion of the gypsum separated by the separator 5 may be added as seed crystals to the Mg(OH).sub.2 regeneration reactor 1.

The objective waste gases to be processed in accordance with this invention can be of various kinds containing, such as SO.sub.2 and/or SO.sub.3, particularly, SO.sub.2. Specific examples are exhaust gases produced by the combustion of sulfur-containing fuels such as, for example, petroleum and coal.

In the practice of this invention in the case of removal of SOx as gypsum, it is possible to cause the quantity of Ca(OH).sub.2 in the absorption liquor to be substantially zero and only Mg(OH).sub.2 to be the absorbent of the SOx, whereby a great increase in the rate of desulfurization of the waste gas is attained, and, at the same time, gypsum of excellent quality is produced. An attendant advantageous feature of the process of this invention is that almost the entire quantity of the gypsum formed in the absorption tower is discharged therefrom, whereby scaling trouble in the absorption tower is eliminated.

In order to indicate more fully the nature and utility of this invention, the following example is set forth, it being understood that this example is presented as illustrative only and that it is not intended to limit the scope of the invention.

The results of tests in a continuous reaction vessel operated on a bench scale are set forth in Table 1. The controlled parameters were as follows.

(1). Percentage of seed crystals fed-- ##EQU1##

(2). Reaction temperature

(3). Mole ratio of double decomposition reactants, MgSO.sub.4 /Ca(OH).sub.2

The concentration of the MgSO.sub.4 in the mother liquor (i.e., the filtrate after separation of gypsum after oxidation) introduced into the Mg regeneration reactor was adjusted to 5 percent.

Each test was carried out by adding a specific quantity of the gypsum seed crystals beforehand in the mother liquor. The Ca(OH).sub.2 added into the Mg regeneration reactor was added in the form of a slurry of 7 percent of Ca(OH).sub.2 thus adjusted beforehand with pure water.

In the series of test runs, with the exception of test run No. 6 wherein Ca(OH).sub.2 remained after the reaction because the double decomposition mole ratio was less than 1 (unity), the Mg(OH).sub.2 in the sampled slurry liquor was neutralized with 98-percent H.sub.2 SO.sub.4 and dissolved while the sample slurry was agitated with a stirrer. Thereafter, this slurry was suction-filtered, and the solid content CaSO.sub.4.2H.sub.2 O was dried at approximately 40.degree. C., after which measurement of particle size was carried out. The distributions of particle size of the gypsum seed crystals and the gypsum obtained by the test runs are set forth in Table 2. The measurement of these particle size distributions was carried out by sieving for sizes above 44 microns and by the optical scanning method for sizes below 44 microns.

(1) Effect of seed crystal feeding rate

When the distributions of particle size of the gypsum formed with the mole ratio of the double decomposition reactants and the reaction temperature controlled at constant values in the case where seed crystals were added and in the case where they were not added, that is, in the cases of test runs No. 1 and No. 2 and test runs No. 4 and No. 5, are examined from photomicrographs (FIGS. 2-5), it is clearly observed that the particles in the case where seed crystals were added had grown to greater sizes than those in the case where the seed crystals were not added. Furthermore, in the case where the seed crystals were added, the number of fine particles of gypsum was overwhelmingly small.

(2) Effect of the reaction temperature

The effect of the reaction temperature on the gypsum particle size, under the condition of constant rate of feed of seed crystals and constant mole ratio of double decomposition reactants, can be compiled as set forth in Table 3.

It is apparent from these results that the higher the reaction temperature, the greater is the growth of the crystals and the greater is the particle size.

(3) Effect of the double decomposition mole ratio

When the test run Nos. 4 and 6 in which the double decomposition mole ratios are respectively 1.25 and 0.67 with the same seed crystal feeding rate and the same reaction temperature are compared by examining their respective photomicrographs, it is observed that test run No. 6 (FIG. 6) exhibits a greater number of crystals of small particle size.

In the case where the double decomposition mole ratio was less than 1.0, Ca(OH).sub.2 remained, whereby test run Nos. 6 and 4 could not be compared with respect to particle size distribution by sieving or by the optical scanning method.